A burner block is a device used to heat either a surface or an article. In general, a burner block is manufactured from refractory material and has a bore therethrough. Air and a combustible gas are introduced into one end of the bore, wherein they are mixed to form a fuel mixture, then ignited. The resulting flame that exits the opposite end of the bore is directed to the surface or article to be heated.
Burner blocks are used in a wide range of industrial applications. For example, burner blocks are used in metal plating operations to diffuse a metal coating into steel. In this process, a steel strip is passed through a molten metal coating bath, such as a bath of molten zinc alloy, to coat the steel strip with the molten metal. The coated strip of steel then is passed between several operating burner blocks which direct flames toward the steel strip. The flames heat the surfaces of the coated steel strip to about 2300.degree. F., and thereby diffuse the metal coating into the steel surfaces.
Conventional burner blocks are constructed from a hard, castable refractory material, such as the material used in the making of refractory concrete, to provide a burner block having a density of about 150 lbs/ft.sup.3 (pounds per cubic foot). Conventional burner blocks fail relatively rapidly because the blocks exhibit severe radial cracking, i.e. cracks that extend perpendicular to the direction of the bore. The radial cracking leads to inefficient burner operation and causes deterioration of the refractories, like insulating fire brick or ceramic fiber modules, surrounding the burner blocks. Burner block deterioration also causes hot spots to form on the furnace shell. As a result, conventional burner blocks must be replaced relatively frequently.
Thermal shock, caused by frequent heating and cooling of the burner blocks, is the major cause of burner block failure. Therefore, improved burner blocks were developed which were manufactured from an upgraded castable refractory, such as a block including an increased amount of alumina or reinforced with stainless steel needles, and that exhibited an improved resistance to thermal cracking. However, such improved burner blocks also eventually failed due to thermal shock within an unacceptably short time period.
Accordingly, there is a need for a burner block that effectively resists radial cracking due to thermal shock and, consequently, is not subject to continual replacement. As an alternative to the conventional castable refractory material, a burner block manufactured from a fiber material has been proposed. The fiber material has an improved thermal shock resistance because the fiber material is internally porous. The fiber material also has the advantage of a substantially reduced density compared to a conventional refractory material, thereby reducing the overall furnace mass and improving furnace response time. In addition, the fiber material exhibits superior insulating properties over conventional refractory material, thereby significantly reducing furnace shell temperatures.
However, a burner block manufactured from a fiber material also possesses substantial disadvantages compared to a burner block manufactured from conventional refractory material. For example, the fiber burner block is brittle, and therefore subject to breaking and crumbling if impacted, such as by a workman during routine maintenance.
An even greater disadvantage demonstrated by a fiber burner block is the relative inability of the fiber material to withstand the high convection environment found in the furnace. When operating, the furnace uses an air-gas fuel mixture that passes through the burner block at a high velocity, such as about 22 ft/sec (feet per second) or greater for a burner having a 3 inch exit port. Usually, the air and gas are mixed in the burner block, with combustion occurring within the block and completed as the fuel mixture exits the burner. The air-gas fuel mixture passing through the burner block at a high velocity creates a turbulence that erodes the fiber furnace block. Erosion occurs even in the absence of particulate matter in the air-gas fuel mixture, but the rate of erosion is greatly increased if particulate matter is present in the air-gas fuel mixture. For example, if particulate matter is present in the air-gas fuel mixture, the fiber burner block is eroded even when the fuel mixture passes through the burner block at a relatively low velocity of about 5 ft/sec.
Accordingly, it would be desirable to provide a burner block that possesses the advantageous properties both of (1) a fiber material burner block, e.g. relatively lightweight, thermal shock resistance and high insulation, and of (2) a refractory material burner block, e.g. structural integrity and erosion resistance. The present invention is directed to providing such a new and improved burner block.